Have you ever seen a rock move? I mean move independently—not, like, roll after someone kicked it or knocked it over—just sliding across a flat surface of its own accord. With the exception of certain psychedelic experiences, I'm going to guess you haven't. But these rocks exist, and they stumped scientists for decades.

The Racetrack Playa is a wide, flat, desert basin in the middle of Death Valley National Park. It's surrounded by tall, rocky hills and, hundreds of years ago, was filled with water, but these days it's just a dried up lakebed. It's one of the hottest, driest, and lowest places in North America, and it's home to the "sailing stones:" rocks that move along the playa on their own. It took until 2014 for scientists to conclusively uncover how the stones get from one place to another: an unusual, and unique combination of ice, wind, and sun.

For a more detailed story behind the sailing stones, listen to this week's episode of Science Solved It:

It's rare to see the stones in motion, but there's a clear sign that they've been on the move. Behind the rocks are long, sometimes winding trails in the dirt. Scientists have used GPS units to track to stones' movements, and have proven that they're mobile. They were first publicly documented in the early 1900s, but for decades, nobody could explain how they moved, according to Richard Norris, an oceanographer at University of California San Diego, and one of the researchers who finally solved the mystery of the rocks.

Image: Wikipedia

"The first scientific study was done 1948 and then there was a succession of scientific papers," Norris said. "Every ten years or so somebody published a new paper on the Racetrack, but nobody is basing their scientific career on this kind of work. It's just a really fun sort of mystery."

Norris became aware of the stones when he was just a kid. His uncle, a geomorphologist at UC San Diego, would take class trips out to the playa, and Norris and his cousin would tag along. As adults, and both scientists themselves, Norris and his cousin decided they ought to take a stab at solving how the rocks moved, so they designed an experiment.

Previous research had produced hypotheses about how the rocks moved. Some thought it was exceptionally heavy winds, others believed ice formed around the rocks and caused them to be buoyant. There were also some less scientific theories, such as acoustic levitation—the belief that sound can cause heavy objects to levitate.

To figure it all out, Norris and his cousin attached specially-designed GPS units to the back of rocks they had brought into Death Valley (the National Parks Service wouldn't let them mess with the rocks that were already there). They also installed a weather station, and then...they waited.

It took two years, but finally, the rocks moved. Norris and his cousin, completely by chance, actually got to witness them in action. The researchers discussed their findings in a paper published in PLOS One. They found that when enough rain fell on the playa to pool, and the temperature dropped, the water would freeze into huge, thin sheets of ice around the rocks—which tumble onto the playa from a nearby hillside. As the morning sun began to melt the ice, if a gentle breeze blew, it could move the ice, which dragged the rocks along with it.

"The ice is like the thickness of a window pane," Norris told me. "And although it's very thin, it's a huge, huge sheet of ice. It's sort of being moved sort of inextricably by these breezes and it can shove around really big things—and a lot of rock."

The results shocked Norris and many other scientists, but it's a phenomenon that's since explained similar moving rocks around the globe. Though it wasn't the paranormal explanation that so many people had hoped for, the truth was even more tantalizing.

"Science is delightful," Norris said.

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